This is a Bug Report

Description

A validation error is thrown when nesting a variable using a custom variable syntax.
This error was discovered when I tried to nest a variable inside a cloud formation reference as such: ${{cf:other-stack-${{self:provider.stage}}.Resource}}. The custom variable syntax used is shown in the serverless.yml file below.

service: addon
provider:
  name: aws
  runtime: python2.7
  stage: prod
  region: us-west-2
  profile: security
  versionFunctions: false
  variableSyntax: '\${{([\s\S]+?)}}'

plugins:
  - serverless-package-python-functions

package:
  individually: true

custom:
  test: ${{cf:core-${{self:provider.stage}}.multiStackCore}} # Error thrown here
  pkgPyFuncs:
    buildDir: build

The error thrown is

   Serverless Error ---------------------------------------

  1 validation error detected: Value 'core-${{self' at 'stackName' failed to satisfy constraint: Member must satisfy regular expression pattern: [a-zA-Z][-a-zA-Z0-9]*|arn:[-a-zA-Z0-9:/._+]*

  Get Support --------------------------------------------
     Docs:          docs.serverless.com
     Bugs:          github.com/serverless/serverless/issues
     Forums:        forum.serverless.com
     Chat:          gitter.im/serverless/serverless

  Your Environment Information -----------------------------
     OS:                     darwin
     Node Version:           7.8.0
     Serverless Version:     1.18.1

This error does not occur when the default variable syntax is used.

Additional Data

• Serverless Framework Version you're using: 1.18.1
• Operating System: Darwin
• Stack Trace: See above